Reciprocal crossing with a susceptible strain and phenotype analysis of F l , test cross, and F2 progeny for resistance showed that dimethoate resistance in the Mackay strain of the cattle tick Boophilus micro plus was due to an incompletely dominant autosomal gene. The degree of dominance of resistance at LCso was +0' 567; resistance factors for homozygotes and heterozygotes were respectively 221 and 69.
Resistance to the organophosphorus acaricides diazinon, dimethoate and formothion in the Biarra (B), Mackay (M) and Ridgelands (R) strains respectively of the cattle tick B. microplus has been shown previously to be controlled in each strain by a single incompletely dominant autosomal genetic factor. A very similar mode of inheritance of fenthion resistance in strain B has now been demonstrated with no departure in degree of dominance of resistance from the mean value of + 0�57 common to these strains exposed to these chemicals. No F 1 larval progeny from the following crossings were appreciably more resistant than their parents to these chemicals: R x B-bromophos ethyl and fenthion; B x M-carbaryl, chlorfenvinphos, chlorpyrifos, diazinon, dimethoate, ethion, fenthion and formothion; M x R-chlorfenvinphos, diazinon, dimethoate, ethion, formothion. The field importance of this absence of overdominance is discussed.
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